The present invention relates in general to processing substrates such as semiconductor wafers, and particularly to preparing a semiconductor wafer before the application of a die coat.
Both high density and lower density integrated circuits are fabricated on wafers utilizing numerous fabrication techniques, including, but not limited to, photolithography, masking, diffusion, ion implantation, etc. After the wafers are fabricated, with the wafer including a plurality of integrated circuit dies, a die coat is commonly used to protect the plurality of integrated circuit dies from damage during the remainder of the manufacturing process. It is commonly known to use polyimides as the buffer or die coat when fabricating such devices or wafers.
Polyimides utilized as a spin-on die coat are somewhat expensive. Typically, conventional processes for applying a die coat require about 5 grams of die coat polymer per wafer. When one considers the volume of wafers processed over, for example, one year, this is a significant amount of material and a significant part of the cost of processing. What is needed is a way to reduce the amount of die coat polymer used without significantly impacting process latitudes. One reason retaining process latitudes is important is because wafer-to-wafer repeatability is enhanced by wider process latitudes.
The goal of the coating step is to produce a generally uniform, adherent, defect-free polymeric film of desired thickness over the entire wafer. Spin coating is by far the most widely used technique to apply such films. This procedure is carried out by dispensing a liquid die coat polymeric composition onto the wafer surface, and then rapidly spinning the wafer until the die coat is distributed over the entire wafer surface.
The spin coating procedure begins with dispensing the polymeric composition onto the wafer. The dispensing stage could be accomplished by flooding the entire wafer with polymeric composition (requiring much more than 5 grams of material per wafer), or by dispensing a smaller volume of polymeric composition at the center of the wafer. The wafer is then brought to a constant spin speed to distribute the composition evenly over the wafer surface. During conventional processing, the wafers are typically subjected to an intermediate ramp spin speed followed by a final spin speed. Film thickness typically depends on the viscosity of the solvent-containing liquid polymeric composition and the final spin speed and time duration of the spin. Once the die coat polymeric composition is distributed, edge bead removal solution is added to the wafer and spun to remove excess die coat material from the wafer. Immediately following the application of the die coat material and the bead removal solution, the wafer is dried or xe2x80x9csoftbakedxe2x80x9d by a heat source such as a hotplate.
The present invention provides a method for treating a substrate, such as a semiconductor wafer, with at least one organic liquid (e.g., organic solvent) prior to depositing a polymeric coating, such as a die coat polymer thereon. Significantly, for preferred embodiments of the present invention, the use of a solvent prewet (as this pretreatment step is referred to herein) reduces the amount of polymer (e.g., die coat polymer) by more than about half compared to a process that does not use a solvent prewet. For example, this solvent prewet process achieves very good polymeric coating thickness profiles (e.g., both wafer to wafer and across wafer), with less than about 5 grams, and more preferably, with less than about 3 grams, and most preferably, with only about 1 gram to about 2 grams, of polymeric material per standard wafer. The method of the present invention preferably results in a very clean film, with good uniformity, with the use of significantly less polymer.
It has been shown that material costs can be reduced and wafer-to-wafer repeatability can be enhanced for a photoresist coating through the use of a solvent prewet. In this system, a solvent is initially spun onto a wafer followed by a photoresist composition. The photoresist composition, which retains essentially the same chemical composition throughout processing, differs from a die coat composition in that the die coat composition includes a xe2x80x9cprecursorxe2x80x9d material that is converted to a polymeric coating. For example, in typical polyimide die coat systems, the precursor is a polyamic acid that is converted to a polyimide after a high temperature cure. Additionally, the photoresist composition has a very low viscosity relative to a die coat polymeric composition. Typically, the ratio of viscosities of a die coat polymeric coating composition to a typical photoresist coating composition is about 100:1. Furthermore, a photoresist material is removed, whereas a die coat material remains on the wafer. Thus, it is unexpected that a solvent prewet would not adversely affect the adhesion of a die coat material.
In a preferred embodiment of the present invention, at least one organic liquid (e.g., organic solvent) is dispensed on a first major surface of a substrate (e.g., wafer) and spun to cover the surface prior to dispensing and spinning a die coat composition on the substrate surface. Preferably, the solvent or solvent mixture is compatible with the polymeric (preferably, die coat) material employed in the over coating (e.g., die coat). As used herein, the term xe2x80x9ccompatiblexe2x80x9d means that the organic liquid (e.g., solvent) does not interfere with the performance properties of the polymeric material or conversion of a polymeric precursor to a polymeric material. Such properties include, for example, adhesion, film retention, etc. A preferred organic liquid for the prewet in the present invention is one that is used in the formulation of the polymeric (preferably, die coat) composition. The organic liquids typically used have a relatively low vapor pressure, e.g., less than about 100 mm Hg at 20xc2x0 C., although higher vapor pressure liquids can be used. The use of a compatible organic liquid (and preferably, a low vapor pressure solvent) for pre-treating the surface in this manner helps to maintain generally equal adhesion properties with much less surface tension. The net result is a clean film coating with significant uniformity achieved using less polymeric material than conventional methods (without a solvent prewet step).
In one specifically preferred embodiment the present invention provides a method of coating a wafer including dispensing at least one organic solvent on a first major surface of the wafer; spinning the wafer until the solvent is distributed across the wafer surface; dispensing a die coat composition on the wafer having solvent on its surface; and spinning the wafer until the die coat composition is distributed across the wafer surface.
In another embodiment, a method is provided for coating a substrate that includes dispensing at least one organic liquid on a first major surface of the substrate; spinning the substrate until the organic liquid is distributed across the substrate surface; dispensing a polymeric precursor composition on the substrate having organic liquid on its surface; and spinning the substrate until the polymeric precursor composition is distributed across the substrate surface; wherein the polymeric precursor composition is converted to polymeric coating that is not subsequently removed from the substrate.
In yet another embodiment, a method is provided that involves incorporating an additional nozzle into a wafer processing machine; positioning the additional nozzle such that it is directed at the center of a wafer held in the wafer processing machine; causing the nozzle to dispense at least one organic solvent on the wafer by triggering a solenoid; spinning the wafer until the organic solvent is distributed across the wafer surface; dispensing a die coat composition on the wafer having organic solvent on its surface; and spinning the wafer until the die coat is distributed across the wafer surface.
In still another embodiment is a method that involves dispensing at least one prewet solvent on a first major surface of a wafer; spinning the wafer to distribute the prewet solvent across the wafer surface; dispensing a die coat composition comprising at least one die coat composition solvent on the first major surface of the wafer having at least one prewet solvent thereon; spinning the wafer to distribute the die coat composition across the wafer surface; and dispensing at least one edge bead removal solvent on a second major surface of the wafer and the edge of the wafer to remove die coat composition present on the second major surface and the edge of the wafer; wherein at least one prewet solvent is the same as at least one die coat composition solvent and at least one edge bead removal solvent.
These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims.